1. The Field of the Invention
This invention relates to rotating wing aircraft, and, more particularly to rotating wing aircraft relying on autorotation of a rotor to provide lift.
2. The Background Art
Rotating wing aircraft rely on a rotating wing to provide lift. In contrast, fixed wing aircraft rely on air flow over a fixed wing to provide lift. Fixed wing aircraft must therefore achieve a minimum ground velocity on takeoff before the lift on the wing is sufficient to overcome the weight of the plane. Fixed wing aircraft therefore generally require a long runway along which to accelerate to achieve this minimum velocity and takeoff.
In contrast, rotating wing aircraft can take off and land vertically or along short runways inasmuch as powered rotation of the rotating wing provides the needed lift. This makes rotating wing aircraft particularly useful for landing in urban locations or undeveloped areas without a proper runway.
The most common rotating wing aircraft in use today are helicopters. A helicopter typically includes a fuselage, housing an engine and passenger compartment, and a rotor, driven by the engine, to provide lift. Forced rotation of the rotor causes a reactive torque on the fuselage. Accordingly, conventional helicopters require either two counter rotating rotors or a tail rotor in order to counteract this reactive torque.
Another type of rotating wing aircraft is the autogyro. An autogyro aircraft derives lift from an unpowered, freely rotating rotor or plurality of rotary blades. The energy to rotate the rotor results from a windmill-like effect of air passing through the underside of the rotor. The forward movement of the aircraft comes in response to a thrusting engine such as a motor driven propeller mounted fore or aft.
During the developing years of aviation aircraft, autogyro aircraft were proposed to avoid the problem of aircraft stalling in flight and to reduce the need for runways. The relative airspeed of the rotating wing is independent of the forward airspeed of the autogyro, allowing slow ground speed for takeoff and landing, and safety in slow-speed flight. Engines may be tractor-mounted on the front of an autogyro or pusher-mounted on the rear of the autogyro.
Airflow passing the rotary wing, alternately called rotor blades, which are tilted upward toward the front of the autogyro, act somewhat like a windmill to provide the driving force to rotate the wing, i.e. autorotation of the rotor. The Bernoulli effect of the airflow moving over the rotor surface creates lift.
Various autogyro devices in the past have provided some means to begin rotation of the rotor prior to takeoff, thus further minimizing the takeoff distance down a runway. One type of autogyro is the “gyrodyne,” which includes a gyrodyne built by Fairey aviation in 1962 and the XV-1 convertiplane first flight tested in 1954. The gyrodyne includes a thrust source providing thrust in a flight direction and a large rotor for providing autorotating lift at cruising speeds. To provide initial rotation of the rotor, jet engines were secured to the tip of each blade of the rotor and powered during takeoff, landing, and hovering.
Although rotating wing aircraft provide the significant advantage of vertical takeoff and landing (VTOL), they are much more limited in their maximum flight speed than are fixed wing aircraft. The primary reason that prior rotating wing aircraft are unable to achieve high flight speed is a phenomenon known as “retreating blade stall.” As the fuselage of the rotating wing aircraft moves in a flight direction, rotation of the rotor causes each blade thereof to be either “advancing” or “retreating.”
That is, in a fixed-wing aircraft, all wings move forward in fixed relation, with the fuselage. In a rotary-wing aircraft, the fuselage moves forward with respect to the air. However, rotor blades on both sides move with respect to the fuselage. Thus, the velocity of any point on any blade is the velocity of that point, with respect to the fuselage, plus the velocity of the fuselage. A blade is advancing if it is moving in the same direction as the flight direction. A blade is retreating if it is moving opposite the flight direction.
The rotor blades are airfoils that provide lift that depends on the speed of air flow thereover. The advancing blade therefore experiences much greater lift than the retreating blade. One technical solutions to this problem is that the blades of the rotors are allowed to “flap.” That is, the advancing blade is allowed to fly or flap upward in response to the increased air speed thereover such that its blade angle of attack is reduced. This reduces the lift exerted on the blade. The retreating blade experiences less air speed and tends to fly or flap downward such that its blade angle of attack is increased, which increases the lift exerted on the blade.
Flap enables rotating wing aircraft to travel in a direction perpendicular to the axis of rotation of the rotor. However, lift equalization due to flapping is limited by a phenomenon known as “retreating blade stall.” As noted above, flapping of the rotor blades increases the angle of attack of the retreating blade. However, at certain higher speeds, the increase in the blade angle of attack required to equalize lift on the advancing and retreating blades results in loss of lift (stalling) of the retreating blade.
A second limit on the speed of rotating wing aircraft is the drag at the tips of the rotor. The tip of the advancing blade is moving at a speed equal to the speed of the aircraft and relative to the air, plus the speed of the tip of the blade with respect to the aircraft. That is equal to the sum of the flight speed of the rotating wing aircraft plus the product of the length of the blade and the angular velocity of the rotor. In helicopters, the rotor is forced to rotate in order to provide both upward lift and thrust in the direction of flight. Increasing the speed of a helicopter therefore increases the air speed at the rotor or blade tip, both because of the increased flight speed and the increased angular velocity of the rotors required to provide supporting thrust.
The air speed over the tip of the advancing blade can therefore exceed the speed of sound even though the flight speed is actually much less. As the air speed over the tip approaches the speed of sound, the drag on the blade becomes greater than the engine can overcome. In autogyro aircraft, the tips of the advancing blades are also subject to this increased drag, even for flight speeds much lower than the speed of sound. The tip speed for an autogyro is typically smaller than that of a helicopter, for a given airspeed, since the rotor is not driven. Nevertheless, the same drag increase occurs eventually.
A third limit on the speed of rotating wing aircraft is reverse air flow over the retreating blade. As noted above, the retreating blade is traveling opposite the flight direction with respect to the fuselage. At certain high speeds, portions of the retreating blade are moving rearward, with respect to the fuselage, slower than the flight speed of the fuselage. Accordingly, the direction of air flow over these portions of the retreating blade is reversed from that typically designed to generate positive lift. Air flow may instead generate a negative lift, or downward force, on the retreating blade. For example, if the blade angle of attack is upward with respect to wind velocity, but wind is moving over the wing in a reverse direction, the blade may experience negative lift.
The ratio of the maximum air speed of a rotating wing aircraft to the maximum air speed of the tips of the rotor blades is known as the “advance ratio. The maximum advance ratio of rotary wing aircraft available today is less than 0.5, which generally limits the top flight speed of rotary wing aircraft to less than 200 miles per hour (mph). For most helicopters, that maximum achievable advance ratio is between about 0.3 and 0.4.
In view of the foregoing, it would be an advancement in the art to provide a rotating wing aircraft capable of vertical takeoff and landing and flight speeds in excess of 200 mph.
A helicopter rotor can be operated at controlled rotational speeds by external airflows only. For example, without the additional power added to the shaft rotating the rotor blades, the rotor blades or rotary wings can autorotate, operating like a windmill. However, autogyros typically, a helicopter uses a power rotor, which therefor has the rearward portion of its operating disk (the theoretical plane in which the blades rotate) upward, with the front portion relatively downward in order to both lift the aircraft up and draw it forward. In contrast, autogyros typically operate with the rotor disk in opposite configuration with the upper front edge relatively higher and the trailing edge of the rotor disk relatively lower in order that relative airflow past the rotor tends to windmill or autorotate the rotor. Thus, the rotary wing provides both windmill autorotation to rotate itself, as well as providing the Bernoulli effect of lift over the airfoil shape of each rotor blade. Thus, at least a portion of the blade or airfoil is dedicated to or responsible for providing autorotation, and at least a portion of the blade is providing airfoil lifting force. Therefore, in an autogyro, forward propulsion is provided typically by a propellor or other device separate from the rotary wing. In contrast, helicopters provide both forward propulsion and lift through the rotary wing.
While rotor speed control at advanced ratios substantially below 1 is straightforward with conventional rotor controls, as the aircraft speed increases, the retreating blades are increasingly exposed to the relative airspeed of the vehicle fuselage. At an advance ratio of 1, the tip speed, the relative velocity of the retreating rotor blade at its extreme end is effectively stationary. That is, for example, the fuselage is traveling forward at a velocity, into the air, while the retreating blade tip is rotating in the opposite direction, at the same speed relative to the aircraft fuselage. Accordingly, the blade tip is effectively stationary. At advance ratios greater than 1, the relative airspeed of the fuselage is such that the trailing edge of the retreating blade is actually exposed to airflow in a reverse direction, that is, from trailing edge toward leading edge.
It would be an advantage to provide some additional, even a relatively small value, of controllable power in order to maintain desired rotor speed at all times. For example, increased lift, typically comes from a combination of collective pitch or the blade angle of attack and a control plane angle of attack. The direction for increasing lift is a function of collective pitch for a rotor trimmed to 0 flapping motion begins to reverse at an advance ratio of 1. The amount of flapping per degree of control angle of attack becomes extremely sensitive above advance ratios of 1. Thus, control may be particularly sensitive at advance ratios that are substantially higher than 1, if the rotor is to be kept in autorotation.
There is a substantial contribution to drag resulting when a rotor is driven in conventional autorotating mode. Therefore, it would be an advantage at higher speeds, particularly where a fixed wing portion of an aircraft may be more effective as a lift mechanism, to still maintain the rotor blades in autorotation at a sufficient speed or angular velocity to maintain their stiffness due to centrifugal forces. Therefore, it would be an advance in the art to provide some mechanism whereby additional autorotating force may be applied to a rotary wing, specifically to an autogyro or autorotating wing, without substantially increasing drag on the aircraft or on the wing, and without increasing fuel consumption, such as would result from powering the rotor or the like.